Steam reforming reactors, hydrodesulfurizers, and the like often require that the hydrocarbon fuel to be processed be introduced into the apparatus in vaporous or gaseous form. In prior art processes liquid hydrocarbon fuel has been vaporized using an external burner to boil the liquid fuel. The life of these boiler units is short due to the buildup of fuel residues (i.e. carbon) on heat transfer surfaces when temperatures exceed 600.degree. to 900.degree. F.
U.S. Pat. No. 2,893,359 shows a relatively complex apparatus for vaporizing corrosive liquids (as opposed to liquid hydrocarbons) such as halogen and metal halides wherein the liquid is sprayed over and falls downwardly through a bed of porous material while a burner supplies hot combustion gases to the lower part of the bed for upward (countercurrent) flow through the bed to impart heat to the liquid and to the packing material in the bed. The hot gases vaporize a portion of the liquid flowing downwardly through the bed. Liquid which is not vaporized falls into a pool below the bed and is recirculated through the bed. The vapors from the liquid and the spent combustion gases are discharged from a conduit above the bed. Aside from its complexity, this vaporization system is not suitable for vaporizing liquid hydrocarbon fuels, since a portion of the fuel would be combusted upon encountering the burner flame at the lower end of the packing column and the afterburner flame above the packing column.
There are numerous patents describing the fractional distillation and separation of liquid hydrocarbons which include vaporization of the liquid feedstock. Some examples are U.S. Pat. Nos. 968,760; 1,523,497; 2,707,163; and 2,779,315. In these processes the apparatus typically comprises a vertical column containing a plurality of beds of packing material vertically spaced apart. The liquid hydrocarbon is boiled at the bottom of the column such that fuel vapors rise up the column and are condensed on the packing material. Heavier hydrocarbons condense out on packing material near the bottom of the column while lighter hydrocarbons rise further up the column where temperatures are lower. (Note in U.S. Pat. Nos. 968,760 and 1,523,497 that condensate on the walls of the conduit is redirected toward the center of the bed by annular deflectors.) As the condensate falls to lower levels, portions are revaporized by the countercurrent upward flow of gas which is hotter near the bottom of the column. (The heaviest portions of the fuel are usually not vaporized, but are collected at the bottom.) In this manner vapors of different hydrocarbons may be drawn off over the length of the column.
While these processes may be well suited for distillation and fractionation as a pure fuel vaporizer they have several disadvantages. First, vaporization is still accomplished by boiling the hydrocarbon, and this results in residue or carbon buildup on hot surfaces which necessitates occasional shutdown for cleaning. Second, countercurrent flow between the rising hot gas and the falling liquid is not conducive to efficient vaporization over a wide range of fuel flow rates. This is because the upward flow of gas has a tendency to entrain liquid fuel droplets before they are vaporized and carry them out of the conduit along with vaporized fuel. Entrainment becomes more of a problem at higher fuel and gas flow rates. High upward gas flow rates also tend to dry out the lower portions of packing regions, overloading the upper regions which don't have the surface area to accommodate all the liquid fuel. The result is that liquid fuel may be blown out the top of the vaporizer.